The present invention relates to a laser device comprising at least one large area vertical cavity surface emitting laser (VCSEL) and at least one optical feedback element providing an angular-selective feedback for laser radiation emitted from said laser. The invention also refers to a method of stabilizing the laser emission of a large area vertical cavity surface emitting laser by angular-selective optical feedback.
High power diode lasers show an increasing number of applications in material processing like cutting, welding or soldering as well as in medicine. Also vertical cavity surface emitting lasers now reach higher output power per device and offer easy scalability in form of 2-dimensional arrays.
The emission of lasers is usually described in terms of Gaussian modes and most lasers are designed to have a fundamental Gaussian intensity distribution. For many laser applications however, especially in material processing or medical applications, different intensity distributions are required in the working plane, e.g. top-hat circular or rectangular shapes or ring structures. Beam homogenizers consisting of one or several lens arrays and at least one Fourier lens can be applied to shape the laser beam to the desired intensity distribution, but have to be carefully aligned in the beam. Furthermore, due to the coherence of the laser beam unwanted artifacts can appear. In addition, ring profiles can only be realized with a significant power loss by the beam homogenizers. Especially such ring shaped intensity profiles can be advantageous in material processing and medical applications. For example, ring profiles can be used in plastic welding to avoid scanning the laser beam on a circular weld seam. Another example is the heating of materials with a high thermal conductivity. If the laser beam used has a top-hat intensity distribution, the temperature distribution on the surface of the workpiece is peaked in the middle due to the better cooling of the rim. With a matched ring shape of the laser beam, the middle peak can be avoided, heating a circular area homogenously.
Small area VCSELs are known to emit Gauss modes. Higher output powers can be achieved by larger active areas, but then the emission changes to distributions best described by Fourier modes. With such large area VCSELs the emission angle of laser radiation depends on the detuning of cavity resonance and optical gain peak as well as on several etalon effects. Large area VCSELs tend to emit mainly in one or more distinct angles depending very sensitive on temperature, driving current and design of the laser. This dependence however results in a shift of the far field with changing current or temperature, as the detuning will change.